Unless otherwise indicated herein, the materials described in this section are not necessarily prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. The disclosure herein relates to an image reading device and an image forming apparatus, and more specifically to a technology for detecting dirt in a document reading position.
Image reading devices called sheet-through devices may be configured to read a document conveyed to a document reading position by a document conveying unit. In some instances, the document may be read through a document glass disposed in the document reading position by using an image reading unit such as a scanner. In image reading devices of this type, dirt such as dust on the document glass may appear as vertical stripes on an image read by the image reading unit, possibly resulting in a defective image produced. The following four techniques for detecting dirt on a document glass are available.
The first technique is a technique for detecting a defective pixel. Detecting the defective pixel may be based on image data obtained by reading a white reference plate and image data obtained by reading a black reference plate through a document glass on which no document is placed.
The second technique is a technique in which a conveyed document is read at different positions in the sub-scanning direction to obtain image data. As such, it may be determined that dirt is present on the document glass if a pixel and an adjacent pixel have significantly different pixel values. Different pixel values may be detected in image data obtained at the leading end of the document and image data obtained at the trailing end of the document.
The third technique is a technique for detecting dirt on the document glass by using an image reading unit that simultaneously reads a plurality of lines.
The fourth technique is a technique for detecting dirt on the document glass by using a color line sensor as an image reading unit.
In the first technique, however, it is difficult to determine whether a defective pixel is caused by dust deposited on a document glass or a guide plate. In the second technique, it is possible to detect dirt arising from dust deposited on the document glass. However, the detectable dirt is limited to dirt arising from dust deposited on the document glass and it is difficult to detect dirt arising from floating dust moving over the document glass.
In the third and fourth techniques, it is also difficult to detect dirt arising from floating dust moving over the document glass. Additionally, the third and fourth techniques are difficult to implement with a monochrome line sensor or a contact image sensor (CIS) configured to read a color image by sequentially switching colors of a light source. Thus, a line sensor may be required such as a color charge-coupled device (CCD) sensor configured to simultaneously read three colors or a line sensor used for dust detection, which may be configured to simultaneously read a plurality of lines. Hence, image reading devices that apply the third and fourth techniques are expensive.